I'll repost what I wrote in your other post (that one seems to have been hijacked).

Could be a boost leak. Check all your hoses and your Blow-off valve (BOV). A loose hose that slipped off or a cut/slice in a hose will let the boost out and the engine won't be able to make full power/boost/rpm/speed.

Do you hear a loud hissing or honking sound when you try to go full throttle?

It could also be a bad wastegate solenoid. If it's failed in the closed/unpowered state... then you won't get above ~6psi max boost... which when I've done it... is around 5300 RPMs... and about 40-ish mph. So... that could be a real possibility.